This invention relates to improvements and innovations in transparentized paper and methods of making the same. Transparentized paper, sometimes known as tracing paper of vellum, has long been used as a drafting medium by architects, engineers and draftsmen. More recently, with the advent of computer assisted drafting, it is used in xerographic copiers and in computer driven xerographic laser and pen plotters. For xerographic applications it is essential that the paper be free of traces of solvent used in the transparentizing process. Such solvents cause severe damage to the organic photoreceptors used on the drums in xerographic machines. Among other properties required, a high level of translucency is needed in order to facilitate rapid reproduction of the drawing by those repographic processes using transmitted light.
In current practice, the required level of translucency is achieved by impregnating the paper, usually a 100% cotton fiber sheet, with a resin whose refractive index is close to that of the cotton fibers. Thus the resin partially fills the interstices between the fibers in the sheet, reducing the number of interfacial refractions a light ray must undergo in traversing the paper. The impregnation is usually accomplished by dipping the paper in a solution of an appropriate resin in an organic solvent, rerolling the paper containing the resin and solvent, allowing it to remain in pack for from several hours to several days, then removing the solvent by drying with heat. Heretofore, the resins and resin blends used for transparentizing have involved at least some of the class of hydrocarbon resins requiring hydrocarbon solvents for their solution. These solvents are difficult or impossible to remove completely from the cellulose fibers or the hydrocarbon resin transparentizing agent. Besides creating an unpleasant odor which is objectionable to the user, the residual retained solvent damages the photoreceptor coating on the drum of the xerographic machine when the paper contacts it. In addition, hydrocarbon solvents are hazardous to handle and are detrimental to the environment.
We have found that it is possible to eliminate residual solvent in the transparentized product by the use of a lower alcohol as a solvent and a mixed acetate isobutyrate sucrose ester as the transparentizing agent. The lower alcohols containing up to 5 carbon atoms are readily released by the completely esterified sucrose at temperatures reached during the drying process. The finished vellum has no odor and has no effect on the photoconductive coating on the xerographic drum.
A further advantage of this combination of solvent and transparentizing agent is the very rapid penetration of the sucrose acetate isobutyrate/alcohol solution into the sheet and the rapid attainment of a uniform distribution within the sheet. The need for a "wet pack" conditioning step is eliminated. The web may thus be impregnated and the solvent removed in one pass, eliminating one time consuming step in the usual process. In addition, the viscosity of the transparentizing solution is sufficiently low that the application to the web may be to one side only and the excess removed by air knife, wire wound rod other doctoring device. The alcohol solvent is readily evaporated with relatively low air velocity and temperature.
The sucrose acetate isobutyrate (Eastman Chemical) which has been used is a completely substituted sucrose compound with between 2 and 3 acetate groups and 5 and 6 isobutyrate groups. It is in an amorphous state at all accessible temperatures. Although hard at normal temperatures, the viscosity drops rapidly at elevated temperatures. At 160.degree. C. the viscosity is approximately 20 centipoise. This property is responsible for the rapid attainment of uniform distribution within the sheet even after the alcohol has been completely evaporated. In fact, we have found that paper can be transparentized without the use of any solvent by applying the fluidized sucrose acetate isobutyrate at 160.degree. C. to one side of a sheet and subsequently maintaining the temperature at 160.degree. C. or above for approximately 2 minutes to achieve uniform distribution of the transparentizing agent.
It may be noted that the index of refraction of this sucrose acetate isobutyrate is close to that of cellulose (1.45 vs 1.55 for cellulose) which makes it a very efficient transparentizing agent.
It is believed that the above desirable characteristics of sucrose acetate isobutyrate may be explained on the basis of its molecular structure. The hydrophobic character of the compound may be attributed to the absence of any hydroxyl groups on the completely esterified molecule. On the other hand it is suggested that the strongly negative carboxyl groups on the sucrose ester molecule interact with the polar cellulose unit thus contributing to the rapid attainment of uniform distribution within the sheet and to firm bonding with the cellulose fibers.
The object of the invention, generally stated, is the provision of high quality, transparentized papers which are at least substantially free of residual solvents and compatible with the organic photoreceptors used on drums in xerographic machines, and improved methods of economically producing the same.
This object of the invention can be achieved by impregnating a paper web, preferably 100% cotton rag, with a solution of sucrose acetate isobutyrate in a lower alcohol containing up to 5 carbon atoms, preferably isopropanol. After impregnation, practically all of the solvent is removed from the paper so that it retains no appreciable content of the solvent and preferably retains from about 10% to about 50% by weight of sucrose acetate isobutyrate based on the paper content.